The total number of electrons of BH2- is 6e- (3e- in B + 2*1e- in H + 1e-). After you draw the lewis structure, you should have a Boron as the central atom with the two hydrogens bonded with single bonds and a lone electron pair on the central B atom. It should look like this:

. . B / \H H

Because the electron density geometry is trigonal planar (and the molecular shape is bent), the bond angles should be 120 degrees each. However, since a lone electron pair has a high repulsion, it pushes the two single bonds downwards and more towards each other, making the bond angle slightly less than what it should be (120 degrees).